Oxidation inhibitors of u3o8



Patented June 10, 1952 UNITED STATE OXIDATION INHIBITORS OF U308 Irving Sheft, Park Forest, and Sherman M. Fried, Chicago, Ill., assignors to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Application March 13, 1950, Serial N0. 149,446

4 Claims.

This invention relates to oxidation-resistant uranium oxide compositions wherein uranium atoms have a valence state lower than 6. This invention further relates to a process for effecting stabilization of the uranium atoms in uranium oxide compositions in a valence state lower than 6, and especially compositions containing U309.

Previous workers have stated that in the presence of oxygen at pressures of one atmosphere or less, the decomposition at temperatures above formula should not be construed to mean that there is more than one type of uranium atom in U303, because X-ray patterns for U303 show that all the uranium atoms in U303 occupy equivalent positions and thus carry the same average charge which charge is maintained by resonance between the U+ valence and the 11+ valence.

Since U03 is the most volatile of all oxides of uranium, the conversion of U303 to the higher and more volatile U03 under temperature conditions and atmospheric conditions favorable to oxidation, poses certain problems in industrial processes utilizing lower oxides of uranium, such as U303, which latter compound is frequently employed as catalyst or as a refractory. The problem presented by the oxidation of lower oxides of uranium and subsequent volatilization of the uranium as U03 is even more acute where lower oxides of uranium are employed as fuel in neutronic reactors wherein the conversion of U303 to the more volatile U03 and its subsequent loss by U03 volatilization is especially undesirable. In addition to the possible loss of uranium from neutronic reactors by oxidation to the more volatile uranium oxide, such oxidation with the accompanying volatilizatlon also effects substantial migration of uranium as the volatile U03, thereby altering the distribution of uranium in the particular geometry necessary for a neutronic reactor.

It is thus an object of this invention to provide an oxidation-resistant uranium oxide composition wherein the average charge on the uranium atoms is less than 6 and the uranium atoms have a valence state lower than the hexavalent state of uranium as found in U03 and which lower valence uranium oxide composition is resistant to further oxidation even under conditions where 2 oxidation of uranium ions to hexavalent uranium as in U03 would ordinarily occur.

A further object of this invention is to provide a method for inhibiting or at least retarding the oxidation of lower oxides of uranium, wherein the average valence state for the uranium atoms is less than 6, to the more volatile uranium trioxide.

A further object of this invention is to provide a process which will eifect stabilization of uranium oxide compositions in a lattice form of a lower uranium oxide, i. e., a form wherein the uranium atoms favor oxidation states which are at least lower than 6.

Other objects and advantages will be apparent upon further examination of the present application.

We have discovered that oxidation of oxides of uranium wherein the valence state of the uranium is less than 6 is substantially retarded and inhibited by incorporating therewith oxides of heavy metals. We have found that oxides of iron, zirconium, gadolinium, manganese, silver, zinc, bismuth, and thorium inhibit oxidation of lower oxides of uranium; this eifect is also observed, but only to a lesser extent, upon incorporation of the oxides of other metals, such as beryllium, within the uranium oxide crystal structure having uranium in a lower oxidation state. The amount of metal oxide or mixture of metal oxides incorporated in the lower uranium oxide compositions can be varied considerably and the preferred amount ranges between 0.1 and 5% by weight of metal, as the metal oxide, based upon uranium content.

Incorporation of only 0.1 weight per cent iron as iron oxide in U303 effectively retards oxidation; addition of zirconium as zirconium oxide to the extent of 13 weight per cent also prevents the oxidation of U303 to U03. Samples of compositions containing zirconium oxide and U303 remain unoxidized even after exposure to heat and superatmospheric pressures of oxygen for a pericd as long as 8 months. Even when these metal oxides are added to the extent of 26 atomic per cent of metal based upon uranium content, the resultant compositions exhibit a monophasic X-ray pattern similar to the pattern for U303. The composition-containing added zirconium oxide has smaller lattice parameters indicative of replacement of uranium by zirconium in the U303 lattice.

The incorporation with lower uranium oxides of oxides of Fe, Be, Zn, and Bi results in oxidation-resistant compositions, which upon sufficient heating under oxygen pressure at high temperture produce compositions shown by X-ray diffraction to be diphasic; Regardless of the X-ray pattern of the resultant compositions, after oxidation the products efiected by the in corporation of oxides of any of the aforemen 3 tioned metals withstand complete oxidation even when heated in the presence of oxygen under superatmospheric pressures, and the amount of uranium lost by volatilization as a higher oxide is substantially reduced. The addition of oxidation-retardant metal oxides to the lower oxides of uranium also affects the physical properties of uranium oxide, for example, the addition of 4 atomic weight per cent metal as an *oxidationretardant metal oxide to U303 effects a softer and less dense composition than pure U308.

In accordance with the process of this invention, oxides of the oxidation-inhibiting metals are incorporated with lower uranium oxides by dissolving the desired proportion of the oxidation-inhibiting metal i-nthe found the metal oxide or salt, such as nitrate, in nitric acid along with dissolution of uranium or Iuranium compound. V The nitric aci solutions may be prepared separately and mixed. The resultant solution .is then evaporated to dryness and the residue ignited in air at an elevated temperature, e. g., 850.-C. Alternatively, the lower uranium oxide is ground in a mortar together with the oxidation-retardant metal oxide and the comminuted oxides are intimately mixed by simple physical or mechanical means prior to firing at 850 C. n i

The following examples are illustrative of the preparation of the oxidation-resistant compositions and their resistance to oxidation.

EXAMPLE I A sample consisting of 0.05 g. of iron mixed with 6 g. of U308. was dissolved in nitric acid, :2

evaporated, and ignited to oxide by heating at 850 C. Five-hundredths g. of beryllium was incorporated with 6 g. of U308 in an identical manner, while 6 g. of U308 without added metal was similarly treated to prepare a control. All three samples were placed in quartz reaction tubes and heated at 600 C. for about seventy-two hours under an oxygen pressure'of approximately 380 lbs. X-ray analyses of the products showed that the control sample of U308 was completely converted to U03 while the X-ray pattern of the product formed from the sample containing beryllia contained 60% U03 and 40% U308, and the product formed from the sample containing iron oxide showed a diffraction pattern for U308 alone. The product formulae based upon the weight losses obtained by ignition in air of samples weighing approximately 1 g. each, were:

Observed Weight Formulae Losses JJg. For the control sample (1.10 g.) 20. 72 UOmm. For the sample containing beryllia (0.73 g.) 11. 63 "(102.05 For-)the sample containing iron oxide (0.75 6. l8 UOMXs.

EXAMPLE II The ignited samples where heated for thirtyeight hours at 600 C. in presence of oxygen under 360 lbs. of pressure. X-ray analysis of the product and calculations of the formulae from the weight loss of samples of the product on ignition in air showed that the oxidation-inhibition of U308 in the samples was proportional to the amount of iron oxide present.

Several series of tests were run in'which U 03 or compositions containing U308 in combination with other metal oxides were prepared and subjected to oxygen pressure of 380 pounds in a bomb at a temperature range between 600 and 650C. in the same manner as in Example I. The U308- containing compositions were prepared by dissolving -U308 in nitric acid along with the dissolution of either a metal, a metal oxide or the metal nitrate. Specifically, zinc, bismuth, manganese, and iron were used as metal for the dissolution step. adolinium was used as the oxide and zirconium and silver were used as nitrates. The resultant aqueous solutions'of uranyl nitrate and other metal nitrate were heated for removal of water after which the residue was'i'gnited' for conversion to metal oxide composition m wmch uranium was present as U303, Thesecompositions were subjected to the high temperature treatment under'oxygen pressure as statediabove using 6 -g. quantities in some experiments and 1.2- g. quantities in other experiments. Samples of the products were examinedby' X-ray diffraction and the products were identified as shown below inthe table which also presents the weight per cent of metal of other man oxides 'inthe compositions used for the high temperature "oxygen treatment. In some cases the atomic per cent of the metal of the metal oxide in'hibitoiis also shown. Other samples of the products were ignited in air at an elevated temperature "for conversion of any U03 produced to U303 whereby the chemical I [formulae were calculated. These formulae were in substantial agreement with the products as determined by X-ray analysis.

Table METAL or iyrn rlu. oXIDE"moonronkrnp Weight Atomic 5 roductaaDetermlned Kind 5 3? 5 by X-ray pattern 1. l7 4. 2 60% UO3;'40% UsOi. 3. 74 4. 2' 8% U03; 92%Ua03. 1 .30:- 0. 33 175011.-

1 4. 2 8% U03; 92% U102. 1 UeOiv- 1. 63 4. 2 U308. 10 U308- 13. 3 U308.

X Based on U content of product.

The foregoing data show how eflfectively the incorporation of these metal oxides inhibits the oxidation of lower uranium oxides such as U308.

The foregoing examples of the process of the present invention and the compositions prepared thereby are intended to be illustrative rather than limiting in scope. The numerous equivalents and modifications thereof, apparent to those skilled in the art, are included within the scope of the present invention. The appended claims are intended to cover as broadly as possible, in view of the prior art, all those features of novelty disclosed herein. taken either singly or in combina tion, and only the limitations shown in the appended claims are to be imposed upon the scope encompassed by this invention.

What is claimed is:

1. An oxidation-resistant composition comprising an oxide of uranium wherein the valence of the uranium atoms is less than 6 and at least 0.1% by weight, based upon uranium content, of iron as iron oxide.

2. A process for inhibiting oxidation of uranium oxides wherein the valence of the uranium atom is less than 6, comprising intimately mixing with said uranium oxides about 1 weight per cent of iron as iron oxide, based upon uranium content, and igniting the mixture in air.

3. A process for the preparation of an oxidation-resistant composition containing a major proportion of an oxide of uranium wherein the valence of the uranium atoms is less than 6, which comprises providing an aqueous solution containing uranyl nitrate and nitrate of iron suificient to provide between 0.1 and 5% by weight of iron as iron oxide based upon the uranium content, evaporating for removal of water to provide a residue, igniting the residue in air at an elevated temperature to provide said oxidation-resistant composition.

4. The process of claim 3 wherein the amount of iron oxide is 1 per cent by weight of iron based upon uranium content.

IRVING SHEET. SHERMAN M. FRIED.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,837,254 Dew Dec. 22, 1931 2,415,878 Hale Feb. 18, 1947 

1. AN OXIDATION-RESISTANT COMPOSITION COMPRISING AN OXIDE OF URANIUM WHEREIN THE VALENCE OF THE URANIUM ATOMS IS LESS THAN 6 AND AT LEAST 0.1% BY WEIGHT, BASED UPON URANIUM CONTENT, OF IRON AS IRON OXIDE. 